Relative velocity between metal surfaces in sliding contacts must occur at slow speeds in loaded conditions in order to produce hydrodynamic lubrication boundary conditions. Boundary conditions are also pervasive when starting and stopping machinery, as for example, in an automotive vehicle. It is well known to the student of tribology that when metal surfaces are in reciprocating modes of sliding contacts, as in the piston and cylinder of an internal combustion engine, friction and wear are most severe at top dead center (TDC) and bottom dead center (BDC) of the stroke positions of the piston. At these exact points where the piston changes direction, a condition of zero velocity occurs causing metal contact.
This condition is most severe at TDC because lubricating oil on the surface thereof is exposed to combustion temperatures causing unfavorable changes in its viscosity. This condition renders it more difficult to retain oil in the pores of the metal surfaces, accelerating the diminishment rate subsequent to when the liner is wiped clean by the oil wipe rings situated beneath the combustion rings of the piston. In the past, cylinder surfaces have been honed producing a cross-hatching effect, or shot blasted in order to increase the retention rates of the oil film on the surface. These techniques have met with only marginal success because of the combination of operating conditions above described. Also, in hydraulic cylinders, boundary conditions occur for some of the same reasons previously described. The traversing speed of the piston within the cylinder is insufficient in terms of relative velocity to produce an oil film thickness necessary to prevent metal to metal contact, thereby making the presence of boundary conditions unavoidable.
Surface bonded films of solid lubricants on many metal substrates have been evaluated on applications where operating conditions cause friction and wear. This was done in order to improve the performance standards in terms of mechanical failure prevention caused by sliding contact of unseparated surfaces. If surfaces contact, this condition in the presence of a lubricating material is known as a "mixed friction condition." When this situation occurs, the surface coating of the solid lubricant will alter the friction characteristics. However, research has clearly demonstrated that when operating conditions are such to cause boundary conditions to prevail, the surface film of the lubricant is diminished rapidly. The removal of the film in some instances is caused by sliding contacts in conformal modes of contact.
In counterconformal modes of sliding contacts, a molybdenum disulfide (MoS.sub.2) surface coating in a hydrodynamic environment where operating conditions cause boundary contact was observed. The hydraulic forces in the oil wedge that accumulates at a point adjacent the position of line contact causes the oil to infiltrate the relatively porous structure of the surface coating causing the removal of the film to become accelerated by the mechanism of cavitation. Consequently, the solid lubricant provides only intial benefits when used and does nothing to mitigate friction and wear once it becomes removed from the surface.
Pursuant to the art of improving the retention rate levels of a solid lubricant surface coating on a metal surface, many techniques have been used upon the metal surfaces prior to the application of solid lubricant surface coating. Included among these processes are phospatizing, etching, shot peening or shot blasting, vapor blasting and carbo-nitriding. These processes are designed to irregularize the surface of the metal, creating pits or micro-reservoirs into which the lubricant surface coating can be deposited. Regardless of which method is used, the coating applied is still a surface coating and is removed from the surface by the onset of boundary conditions. Also, shot peening or shotblasting subjects the surface to relatively high amounts of compressive and residual stress. A study at the John Deere Product Engineering Center in Waterloo, Iowa, clearly underscored this fact on an oil pump gear. A MoS.sub.2 surface coating was applied to an alloyed steel surface after the gear had been case hardened by means of carbo-nitriding. During a 500 hour comparative wear test, it was observed that the surface film of the solid lubricant was completely removed from the alloyed steel surface after only 43 hours.
In recent research done at the Engineering Research Institute, located on the campus of Iowa State University at Ames, Iowa, it was observed that in a counterconformal mode of sliding contact during the production of boundary conditions, the combination of oil and MoS.sub.2 surface coating caused the surface coating to become abrasive in situ. This abrasiveness caused metal removal to become accelerated from the underlying substrate. This phenomena is in certain aspects puzzling and not completely understood. It has been postulated that it could be caused by the anisotropic crystal structure of MoS.sub.2. Because the MoS.sub.2 has a lattice layer-type structure, the bonds on the basal plane are long and possess a low lamellar shear strength which shear easily on sliding contact. However, in the crystal planes perpendicular to the basal plane, the bonds are much shorter and therefore many times stronger. Recent studies have shown these differences in strength between bonds in the MoS.sub.2 crystal to be on the order of magnitude of 29.
It is thus clear that some method must be devised to increase the endurance life of a solid lubricant used in operating conditions within a hydrodynamic environment causing boundary conditions. Benefits accruing by increased mitigation of friction and wear in terms of the life of the parts would lead to the production of more efficient and economical machines which utilize sliding contacts.